A further 32 suspected cases of swine flu among pupils at Eton College have been reported to health officials.
The public school announced on Thursday it would close for a week after a 13-year-old pupil tested positive.
The Health Protection Agency (HPA) has written to parents warning of a further seven probable and 25 possible cases.
The Berkshire boys’ school, which was attended by Princes William and Harry, was advised to shut for an extra week after half-term until 7 June.
The HPA said of the suspected 32 new cases that students were experiencing “mild illness”.
In a letter to parents the agency said: “It is highly likely that these will be positive for Swine Influenza.
“Because it is now clear that there is widespread transmission in the school, the HPA has decided to offer antiviral [medication] for all the students and staff at the school.
“The purpose of this is to minimise the likelihood of further spread of the virus to students and the families not already affected.”
Boys taking public examinations at Eton are being allowed to return to do so under controlled conditions.